Detailed Description of the Invention
1. Technical Field of the Invention
The present invention relates to a developing device used in an electrophotographic recording apparatus, an electrostatic recording apparatus or the like, for selectively transferring toner on a latent image based on an electrostatic potential difference for visualizing, and more particularly to a developing device using two-component developer obtained by mixing carrier with toner.
2. Related Art
In an electrophotography method, a developing method using the two-component developer containing toner and magnetic carrier has the advantages that it is easy to charge toner and flocculation of toner particles is also difficult to occur. For this reason, it has been more frequently used than before although it is necessary to control an amount of toner contained in the two-component developer, i.e., toner density.
FIG. 25 is a schematic structural view showing a conventional example of a developing device using two-component developer.
This developing device comprises: a developing roll 202 arranged in proximity to an image carrier 201 to face it, for magnetically attracting and conveying developer; a developer regulating member 203 for regulating an amount of developer attracted on the developing roll to provide a substantially uniform magnetic brush; a paddle 204 for supplying developer to the developing roll 202, and two augers 205 and 206 for conveying and agitating developer within a housing 210.
The foregoing developing roll 202 comprises a magnet roll 211 fixedly supported, and a cylindrical sleeve 212 rotationally driven around the magnet roll so that the magnet roll 211 attracts the developer on the sleeve to convey the developer to an opposite portion to the image carrier 201 by the rotation of the sleeve 212.
The foregoing two augers 205 and 206 rotate so as to convey the developer in the directions opposite to each other respectively within two agitation chambers 207 and 208 provided behind the developing roll 202, and the developer is circulated and moved within the two agitation chambers which are conductively connected to each other at their both ends.
In such a developing device, carrier and toner are sufficiently agitated within the agitation chambers 207 and 208, and a part of the developer is supplied to the developing roll 202 by the paddle 204. This developer is attracted on the sleeve 212 by a pickup magnetic pole 213 of the magnet roll 211, and the layer thickness is regulated by a developer regulating member 203, and thereafter the developer is conveyed to a developing area to be used for development.
The developer, which has passed through the developing area, is released from the sleeve 212 by a pick-off magnetic pole 214, and is returned to the agitation chamber by the paddle 204. It is mixed with the other developer and toner newly replenished here to be sufficiently agitated.
An amount of charge of toner in the developer used in such a developing device fluctuates depending upon the environmental conditions, and also greatly fluctuates depending upon the toner density in the developer. FIG. 26 shows relationship between the toner density and the amount of charge of the toner within such a two-component developing device as described above under each environmental condition of high temperatures/high humidities, intermediate temperatures/intermediate humidities, and low temperatures/low humidities. Generally, the amount of toner charge fluctuates with such characteristics as shown in FIG. 26, and the amount of toner charge must be maintained constant in order to obtain fixed development characteristics under each environmental condition. To this end, it is necessary to perform the following control.
That is, when the operating environment changes from high temperatures/high humidities (state indicated by a symbol a in FIG. 26) to intermediate temperatures/intermediate humidities (state indicated by a symbol b' in FIG. 26), toner must be replenished to increase the toner density from A to B so that the amount of toner charge becomes a predetermined value (state indicated by a symbol b in FIG. 26). Also, when the environmental condition changes from low temperatures/low humidities (state indicated by a symbol c in FIG. 26) to intermediate temperatures/intermediate humidities (state indicated by a symbol d in FIG. 26), the toner density must be reduced from C to B so that the amount of toner charge becomes a predetermined value (state indicated by a symbol b in FIG. 26). In the conventional two-component developing device, however, there have had a problem that there is no method for reducing the toner density except consuming the toner, but the amount of toner charge becomes low, causing fog on the background.
Also, in order to control such toner density, a reference image has been actually developed to detect the density, and control, in which toner is replenished among others, has been performed, and a complicated control method and device therefor have been required. Under such circumstances, a developing device, in which an attempt is made to obtain an image with stable density by controlling the toner density by a simple mechanism, is described in, for example, Japanese Published Unexamined Patent Application Nos. Sho 59-111664 and 63-287874, Japanese Published Unexamined Patent Application No. Hei 5-59427, and Japanese Published Unexamined Patent Application No. Hei 7-84456.
A developing device according to the technique described in the Japanese Published Unexamined Patent Application No. Sho 59-111664 comprises: a magnetic conveying part consisting of a magnet roller and a non-magnetic sleeve provided apart from it on an outer peripheral surface thereof; and such a rotatable toner supply roller as to come into contact with bristle of the developer formed on the foregoing non-magnetic sleeve, and the foregoing toner supply roller is caused to abut upon a toner layer thickness regulating member (blade) to form a thin layer of toner charged on the surface of the foregoing toner supply roller. On replenishing toner to the developer on the non-magnetic sleeve from the toner supply roller, a predetermined level of potential difference is imparted between the foregoing non-magnetic sleeve and the foregoing toner supply roller to control an amount of toner movement from the surface of the foregoing toner supply roller to the foregoing non-magnetic sleeve whereby the toner density is caused to be maintained substantially constant.
According to the technique described in the Japanese Published Unexamined Patent Application No. Sho 63-287874, developer conveyed by the developing roll is regulated to a desired thickness by a regulating plate, the developer separated from the developing roll is caused to fall by gravity within a vessel, and this developer which has fallen is conveyed by the developing roll again. Thus, circulation of the developer is formed, and the toner density is caused to be maintained constant by causing toner to intermittently come into contact with the developer circulating.
According to the technique specified in the Japanese Published Unexamined Patent Publication No. Hei 5-59427, bristle of a magnetic brush on the sleeve is caused to slidably contact a mesh screen arranged at the opening of a toner hopper, whereby the toner within the toner hopper is caused to move to the foregoing magnetic brush for self-adjustment of the toner density.
According to the technique specified in the Japanese Published Unexamined Patent Application No. Hei 7-84456, the surroundings of the developing roller are regarded as a narrow space, and the amount of carrier within this space is set to a substantially constant value, whereby the amount of the magnetic toner contained in the remaining space is adjusted to thereby control the toner density substantially constant.
In this respect, there has also been reported in large numbers a device in which the toner density is not adjusted unlike the foregoing, but charging of the toner in the developer is promoted with a simple mechanism. According to the technique specified in, for example, Japanese Published Unexamined Patent Application No. Hei 9-43993, in a device for causing a magnet roller to rotate for conveying developer, a magnet member having a plurality of magnetic poles is caused to oppose to the developer which has passed through the toner supply area, and an alternating magnetic field is formed in the area opposite thereto, to thereby increase the agitating force of the developer.
Problems to Be Solved by the Invention
However, such a developing device as described above has the following problems. In a developing device specified in the Japanese Published Unexamined Patent Application No. Sho 59-111664, a thin layer of toner charged is formed on the foregoing toner supply roller by urging the toner layer regulating member against the toner supply roller. For this reason, a strong frictional force acts between the foregoing toner layer regulating member and the toner, the toner is turned into a film on the foregoing toner supply roller, replenishment of toner from the toner supply roller to the developing roller will not be smoothly performed, or defective charging of toner will occur.
In a developing device specified in the Japanese Published Unexamined Patent Application No. Sho 63-287874, in order to circulate developer, the developer is caused to completely separate from the developing roll, and to fall by gravity, and is raised upwardly by the foregoing developing roller again. Therefore, there are the problems that the developer on the developing roll cannot be agitated quickly, and a considerably wide space is required for the circulation of the developer.
Also, in order to sufficiently agitate the developer circulating, it is necessary to set the magnetic force of the developing roll to be strong, and to that end, there is the problem that deteriorated developer occurs in a portion of shaping the magnetic brush. Also, a great difference occurs in an amount of toner replenished between when a solid image having gradation such as a photograph, a picture, a map and the like is mainly printed, and when a line image is mainly printed. In a system of circulating the developer, however, a mechanical rotary motion of developer causes a fixed amount of toner to be taken in at all times, and therefore there is the problem that the amount of toner supply cannot be varied in conformity with the image printed so that it becomes difficult to maintain the toner density constant.
A developing device specified in the Japanese Published Unexamined Patent Publication No. Sho 5-59427 has the problem that since developer slidably contacts a mesh screen, stress is applied to the developer, leading to much shortened life of the developer. Also, since the fluidity of toner and the charging property of the toner, i.e., an adhesive force between toner and carrier greatly contribute to the control of toner density, there is the problem that the toner density control range will be beyond a range set at the beginning and the printed image quality will be different from that at the beginning if the fluidity and charging property of the toner change depending upon the environment or elapsed time.
Also, a great difference occurs in an amount of toner replenished between when a solid image having gradation such as a photograph, a picture and a map is mainly printed, and when a line image is mainly printed. In the mesh screen system, however, the contact area between carrier and toner is limited, and therefore, it becomes difficult to maintain the toner density constant when the amount of toner thus replenished fluctuates greatly.
Further, toner is replenished to the developer while a magnetic brush is formed on the magnet roll, and since the developer is usually in a flocculated state, the effective contact area of the carrier is reduced, and defectively-charged toner is prone to occur.
In a developing device specified in the Japanese Patent Laid-Open Application No.7-84456, magnetic toner is used and it is necessary to cause toner to contain magnetic powder, but there is the problem that magnetic powder cannot be mixed with color toner because of a problem concerning coloring property and the color toner cannot be used. In a case where great importance is placed on the coloring property and non-magnetic toner is used for the present developing device, any force for attracting toner to the developing roll by means of a magnetic force does not act, but further for a reason that a difference in specific gravity between the non-magnetic toner and the magnetic carrier is large and other reasons, agitation of the non-magnetic toner and the magnetic carrier within a narrow space around the developing roller will not be sufficiently performed. This leads to the problems that the toner is insufficiently charged, and toner cannot be sufficiently replenished to the developer supplied to the development area.
The present invention has been achieved in the light of the above-described problems, and is aimed to provide a developing device capable of promoting toner charging by sufficiently agitating developer, to which toner has newly been replenished, on a developer supplying member, or in addition thereto, controlling the toner density and the amount of toner charge in the two-component developer with simple structure, and obtaining good image quality with stability even when the environmental conditions fluctuate and when the amount of toner used fluctuates depending on a difference in the originals.